Out of curiousity, what is the fluid level at? The trans in my Dakota would slip into neutral when the fluid was low. I had a rusted out pan. Be the first thing I check. I don't know on the rattling though.
Here's a link with the different ratios they offered. Have to scroll down a bit. Looks like the A745 was the heavy duty version.
What manual transmission do I have? - Drivetrain
Definitely take a look at the Dakota for a donor 3.55 or 3.90 LSD. The 3.90 is what is in my 97 Dakota with a 330 hp 318 and it has had zero issues. And it doesn't get driven easy.
Is it the original trans? If it's an A998 or A999 the gear ratios are 2.74 first, 1.54 second and 1.00 third. Might explain the decent off the line performance a bit. And it would be a lockup TC as RRR mentioned.
If you had to a GM product I think the 200 r4 is the better choice, when properly built. The ratios are spread a little more evenly than the 700 r4
________1___2___3___4
200 R4 2.74 1.57 1.00 .67
700 R4 3.06 1.62 1.00 .70
And just for comparison...